Show validation errors when creating external rules

[marc-sift]

Example:

Traceback (most recent call last):
  File "/home/mjulien/code/sift/python/examples/rule_evaluation/external_rules/yaml_config/main.py", line 52, in <module>
    report = rule_eval_service.evaluate_external_rules_from_yaml(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/mjulien/code/sift/python/lib/sift_py/rule_evaluation/service.py", line 191, in evaluate_external_rules_from_yaml
    rule_ids = self._rule_service.create_external_rules_from_yaml(paths, named_expressions)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/mjulien/code/sift/python/lib/sift_py/rule/service.py", line 119, in create_external_rules_from_yaml
    return self.create_external_rules(rule_configs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/mjulien/code/sift/python/lib/sift_py/rule/service.py", line 333, in create_external_rules
    raise Exception("Failed to create external rules\n" + reason)
Exception: Failed to create external rules
asset_expression_validation_results {
  asset_id: "a695480e-0069-44b0-ab4b-93e602f743cb"
  error: "undeclared reference to \'a\' (in container \'\')"
}